PREDICTING THE ONSET OF TRANSFORMATION UNDER NONCONTINUOUS COOLING CONDITIONS .1. THEORY

On cooling a steel, the temperature at which a new phase forms is an i mportant parameter in the genesis of final microstructure. For diffusi onal transformation processes, prediction of this temperature, until n ow, has relied upon empirical equations which are based on the cooling rate or degree of undercooling. Modern steel processing procedures in volve a range of continuous and step cooling schedules, and these equa tions are not appropriate in many situations. A method that is capable of predicting the onset of transformation during the accelerated cool ing of steel, regardless of how complex a thermal path may be, was der ived. The concept of an ''ideal'' isothermal transformation curve for the start of transformation was introduced based on the assumption of the consumed fractional incubation time being additive. Mathematical r elationships between the experimental time to the start of transformat ion and the ideal incubation time were quantified, and methods for der iving an ideal time-temperature-transformation (TTT) curve from experi mental data were established in this study. Details of theoretical der ivations are presented in the first article of this two-part series. A pplication of this prediction method to an austenite-to-pearlite trans formation process was studied to show the validity of the derivation. Experimental procedures and results of this application are given in P artII.